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    <p>Unfortunately, this isn't possible in JavaScript without some intermediate encoding. To understand why, let's assume you base64 decoded and posted the data, like you described in your example. The first few lines in hex of a valid PHP file might look like this:</p> <pre><code>0000000: 8950 4e47 0d0a 1a0a 0000 000d 4948 4452 .PNG........IHDR 0000010: 0000 0080 0000 0080 0806 0000 00c3 3e61 ..............&gt;a </code></pre> <p>If you looked at the same range of hex of your uploaded PNG file, it would look like this:</p> <pre><code>0000000: 8950 4e47 0d0a 1a0a 0000 000d 4948 4452 .PNG........IHDR 0000010: 0000 00c2 8000 0000 c280 0806 0000 00c3 ................ </code></pre> <p>The differences are subtle. Compare the second and third columns of the second line. In the valid file, the four bytes are <code>0x00</code> <code>0x80</code> <code>0x00</code> <code>0x00</code>. In your uploaded file, the same four bytes are <code>0x00</code> <code>0xc2</code> <code>0x80</code> <code>0x00</code>. Why?</p> <p>JavaScript strings are UTF. This means that any ASCII binary values (0-127) are encoded with one byte. However, anything from 128-2047 gets <strong>two</strong> bytes. That extra <code>0xc2</code> in the uploaded file is an artifact of this multibyte encoding. If you want to know exactly why this happens, you can read more about <a href="http://en.wikipedia.org/wiki/UTF-8#Description">UTF encoding on Wikipedia</a>.</p> <p>You can't prevent this from happening with JavaScript strings, so you can't upload this binary data via AJAX without using base64.</p> <p><strong>EDIT:</strong> After some further digging, this is possible with some modern browsers. If a browser supports <code>XMLHttpRequest.prototype.sendAsBinary</code> (Firefox 3 and 4), you can use this to send the image, like so:</p> <pre><code>function postCanvasToURL(url, name, fn, canvas, type) { var data = canvas.toDataURL(type); data = data.replace('data:' + type + ';base64,', ''); var xhr = new XMLHttpRequest(); xhr.open('POST', url, true); var boundary = 'ohaiimaboundary'; xhr.setRequestHeader( 'Content-Type', 'multipart/form-data; boundary=' + boundary); xhr.sendAsBinary([ '--' + boundary, 'Content-Disposition: form-data; name="' + name + '"; filename="' + fn + '"', 'Content-Type: ' + type, '', atob(data), '--' + boundary + '--' ].join('\r\n')); } </code></pre> <p>For browsers that don't have <code>sendAsBinary</code>, but do have <code>Uint8Array</code> (Chrome and WebKit), you can polyfill it like so:</p> <pre><code>if (XMLHttpRequest.prototype.sendAsBinary === undefined) { XMLHttpRequest.prototype.sendAsBinary = function(string) { var bytes = Array.prototype.map.call(string, function(c) { return c.charCodeAt(0) &amp; 0xff; }); this.send(new Uint8Array(bytes).buffer); }; } </code></pre>
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    1. COAh right! Thanks for the explanation! So the question is how do I build the post request so PHP knows I'm trying to send it base64 data and it should decode it? I tried setting `Content-Transfer-Encoding: base64` (thanks @Wrikken!) but like you said it's not a valid header so the browser refuses to do it ("Refused to set unsafe header"). Is there something I can put after `Content-Disposition` that would do it? Been experimenting for a while but haven't found anything that works.
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    2. CO...also, another thing I find confusing here is when I look at a working request generated by the browser itself (from a form with an image input), I don't see any image data. Just `Content-Disposition: form-data; name="formfield"; filename="picture.png" Content-type: image/png` then nothing. Where is this data? Does it mean I should be transferring the actual image data in a separate request or something? Sorry, I'm very new to this kind of stuff and have yet to find a decent tutorial...
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    3. CO@BaronVonKaneHoffen I messed with it for quite a while to try to find a way to get PHP to automatically base64 decode the file. I couldn't find one. :( Are you able to upload any custom PHP to the server? If you can, you could proxy the request through a custom script to decode it.
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